1. Field of the Invention
The present invention relates to a system connecting device for converting DC power into AC power using an inverter and supplying an AC power supply or an AC load with the AC power.
2. Description of the Background Art
With recent development of power generators using alternate energy such as solar cells and fuel cells, there has been an increasing need to develop a device for connecting such DC power generators to an existing power-frequency electric system thereby to cause backflow of generated DC power to the existing electric system. In the case of the solar cells, for example, since DC power is generated, direct connection with the AC power supply or the AC load is impossible. Then, there has been devised a variety of system connecting devices for converting DC power into AC power using an inverter, and causing backflow of the AC power to the AC power supply or supplying the AC load with the AC power.
FIG. 13 shows a conventional system connecting device SC5 using a solar cell. This system connecting device SC5 comprises a solar cell DC1, a power-frequency AC power supply AC1, and an inverter IV5. Nodes N1 and N2 on positive and negative electrodes of the solar cell DC1, respectively, are connected to the inputs of the inverter IV5, whereas nodes N6 and N7 of the AC power supply AC1 are connected to the outputs of the inverter IV5. From a safety standpoint, the node N7 of the AC power supply AC1 is also connected to ground, bearing a fixed potential, e.g., a ground potential GND.
FIG. 14 shows an example of the structure of the inverter IV5. This inverter IV5 is composed of four insulated gate bipolar transistors (IGBT) IT1 to IT4 and four diodes ID1 to ID4. Both collectors of the transistors IT1 and IT2 are connected to the node N1, and both emitters of the transistors IT3 and IT4 are connected to the node N2. The emitter of the transistor IT1 and the collector of the transistor IT3 are connected in common to the node N7, whereas the emitter of the transistor IT2 and the collector of the transistor IT4 are connected in common to the node N6. The anodes of the diodes ID1 to ID4 are connected to the emitters of the transistors IT1 to IT4, respectively, and the cathodes of the diodes ID1 to ID4 are connected to the collectors of the transistor IT1 to IT4, respectively. Each transistor is connected at its gate to a control signal generator (not shown) to receive a control signal for operation of the inverter (e.g., in synchronization with the control signal, the transistors IT1 and IT4 are on during a first half cycle and the transistors IT2 and IT3 are on during a second half cycle). This type of inverter is generally called a single-phase bridge inverter.
Now, we will describe the operation of the inverter IV5. First, control signals are applied to the gates of the transistors IT1 and IT4 so that both the transistors are brought into conduction. At this time, a potential at the positive electrode of the solar cell DC1 (potential on the node N1) becomes the ground potential GND since the positive electrode is connected via the transistor IT1 and the node N7 to ground. A potential at the negative electrode of the solar cell DC1 (potential on the node N2) is lower than the potential at the positive electrode which is the ground potential GND, by a potential generated by the solar cell DC1. Thus, a potential on the node N6 of the AC power supply AC1 that is connected to the node N2 by the transistor IT4 becomes lower than the ground potential GND.
Then, the application of the control signals to the transistors IT1 and IT4 is stopped, and control signals are applied to the gates of the transistors IT2 and IT3 so that both the transistors are brought into conduction. At this time, the potential at the negative electrode of the solar cell DC1 becomes the ground potential GND since the negative electrode is connected via the transistor IT3 and the node N7 to ground. The potential at the positive electrode of the solar cell DC1 is higher than the potential at the negative electrode which is the ground potential GND, by the potential generated by the solar cell DC1. Thus, the potential on the node N6 of the AC power supply AC1 that is connected to the node N1 by the transistor IT2 becomes higher than the ground potential GND.
Repetitions of such switching operation in predetermined cycles by the control signals permits power conversion from DC to AC. The diodes ID1 to ID4 are provided for the purpose of ensuring a feedback path of current when load current fails to flow from the collector to emitter of each transistor during the transition of the switching.
In the system connecting device SC5 of FIG. 13, the AC power supply AC1 is connected to ground but the solar cell DC1 is not. From a safety standpoint, a node at either one of the electrodes of the solar cell DC1 is preferably connected to ground at a fixed potential (e.g., the node N2 is hereinafter referred to as a node to be grounded). In this case, there is no charge/discharge current in electrostatic capacity which is caused between the ground potential GND and a floating potential when the node is not grounded. This improves electric efficiency of the device.
Grounding of the node N2, however, establishes a ground for both the nodes N6 and N7 when the transistors IT1 and IT4 are brought into conduction. This is dangerous to the AC power supply AC1 and makes it difficult to properly transmit the DC power generated by the solar cell DC1 to the AC power supply AC1 as AC power. From these reasons, the system connecting device SC5 as shown in FIG. 13 is available only for use with a simple system that requires no ground connection of the node N2 and delivers only small output power.
Then, a device configuration permitting grounding of both the nodes N2 and N7 has been developed. FIG. 15 shows such a system connecting device SC6. This system connecting device SC6 further comprises a transformer TR2 in addition to the solar cell DC1, the AC power supply AC1, and the inverter IV5 similar to those of the system connecting device SC5 in FIG. 13. The transformer TR2 disposed between the inverter IV5 and the AC power supply AC1 provides electrical isolation, so that both the solar cell DC1 and the AC power supply AC1 can be grounded.
In FIG. 15, a DC voltage regulator DS1 and a capacitor C4 are further provided between the solar cell DC1 and the inverter IV5. The DC voltage regulator DS1 has the function of outputting a constant voltage even if the output voltage of the solar cell DC1 varies with the conditions of solar irradiation. The capacitor C4 has the function of smoothing out the output of the DC voltage regulator DS1. In general, a voltage inverter connected to a voltage source, such as the inverter IV5, has an optimum conversion ratio of the DC to the AC voltage. Thus, conversion at the other ratios may degrade output characteristics of the inverter or may uneconomically increase current capacitance of semiconductor devices used therein. For efficient operation of the inverter IV5, therefore, it is desirable to supply a constant DC voltage.
The input of the DC voltage regulator DS1 is connected via the nodes N1 and N2 to the positive and negative electrodes of the solar cell DC1 and the output thereof is connected via the nodes N4 and N2 to the input terminals of the inverter IV5. The capacitor C4 is connected at its one end to the node N4 which is one input end of the inverter IV5, and at its other end to the node N2.
As the DC voltage regulator DS1, a chopper circuit is for example employed. There are three types of chopper circuits: buck (step-down) type, boost (step-up) type, and buck-boost (step-up/down) type, from which a boost type chopper circuit as shown in FIG. 16 is employed as the DC voltage regulator DS1. This boost type chopper circuit comprises a reactor L1c, an insulated gate bipolar transistor CT1c, and a diode D1c. One end of the reactor L1c, the emitter of the transistor CT1c, and the cathode of the diode D1c are connected to the nodes N1, N2, and N4, respectively. The other end of the reactor L1c is connected via a node N8 to the corrector of the transistor CT1c and the anode of the diode D1c. The gate of the transistor CT1c is connected to a control signal generator (not shown) to receive a control signal for generation of the DC constant voltage (e.g., a pulse signal having constant cycle and phase).
In this boost type chopper circuit, during the transistor CT1c is on, current flows from the node N1 to N2 and electromagnetic energy is stored in the reactor L1c. With the transistor CT1c off, on the other hand, the electromagnetic energy stored in the reactor L1c is drained to the node N4 and stored as electrostatic energy in the capacitor C4 that is connected to the node N4. Thus, by controlling the control signal applied to the gate of the transistor CT1c, the switching of the transistor CT1c is repeated and the length of time that the transistor CT1c is in the ON state is controlled. This permits a supply of predetermined constant DC voltage to the inverter IV5.
In this system connecting device SC6, both the solar cell DC1 and the AC power supply AC1 can be grounded, but the transformer TR2 which is heavy in weight and large in occupied volume becomes necessary. In addition, the number of elements is increased by the presence of the inverter IV5, and for efficient operation of the inverter IV5, the DC voltage regulator DS1 is necessary. Accordingly, development of the system connecting devices that can resolve these problems is desired.
Japanese Patent Laid-open No. 10-14244 and No. 7-213072, for example, disclose techniques of system connecting devices with no transformer. These techniques are, however, not the solutions to the problems of the circuit shown in FIG. 15 in that the former is the structure in which the AC power supply cannot be grounded and the latter is the structure corresponding to a single-phase three-wire system AC power supply. Further, both techniques employ a single-phase bridge inverter, so that the problem of too many transistors and diodes remains. More specifically, the single-phase bridge inverter requires at least four transistors and four diodes. Besides, the input portion of the bridge is generally connected to a DC voltage regulator such as a boost type chopper circuit and each output terminal thereof is provided with one reactor, which further increases the number of elements.